Method and device for multiplexing data signals

ABSTRACT

Before modulating the transmission carrier, each binary data signal is individually coded as a discrete symbol belonging to a PSK constellation of N symbols which are combined with N discrete points to produce two binary signal streams of which the first is in phase with the incoming signal while the second is in phase quadrature with said incoming signal. Each of said two binary signal streams is assigned its own signature by combining it with a discrete spreading code to produce two spread composite signal streams each of which is to be applied to a modulator. Both of said spreading codes are substantially orthogonally cross-correlated. The method may be used in a satellite communications system, in particular for digital sound transmission and communication via land microterminals.

The present invention relates to satellite communication systems and inparticular a Code Division Multiple Access (CDMA) communication systemsuitable for transmitting band-limited data.

Communication satellites are already widely used for data transmission.Now, the substantial possibilities offered by microelectronics areopening up new applications for telecommunication satellites such asdigital sound broadcasting and very small aperture mobile and personalterminal (VSAT) communications. Digital sound broadcasting, inparticular, is a very promising field of application and is gatheringconsiderable interest in the field of telecommunications. The successfulintroduction of these new satellite based communication services,however, is driven by the performance that can be achieved, by the gooduse of the on-board satellite resources and by a clever ground terminalengineering. A key element determining the satellite communicationsystem complexity and efficiency is the network access scheme.

FR-A-8910481 describes a code division multiple access communicationsystem in which the transmission carrier is activated by the user'svoice and in which synchronization is achieved by means of a mastercode. The key feature of this known system is the use of a differentuser terminal synchronizing scheme in the forward and return links. Inthe forward link, synchronization is achieved by broadcastinguninterruptedly a master code. In the return link towards the satellite,synchronization is achieved by an alignment procedure based on a forcedcarrier voice-activation and selection of a user code produced bydecoding an element contained in the master code.

The multiple access scheme implemented in this known system is imposinga code period equal to the symbol duration when it is required toachieve a drastic self-noise reduction. These limitations were discussedby R. De Gaudenzi, C. Elia, R. Viola in "Performance Evaluation OfSynchronous Code Division Multiple Access (S. CDMA) For Satellite MobileSystems", Proceedings of the IEEE Global Telecommunications ConferenceGLOBECOM'90, San Diego, Calif., Dec. 2-5, 1990.

This known method improves the CDMA system efficiency by reducing theacquisition time of the spread spectrum in the receive circuitries.However, two major problems were emerging.

First, it was established that the performance of the CDMA techniquecould be improved by combining the spreading code synchronization, asused in the aforementioned known method, with the use of synchronizedGold codes. However, using a synchronized spreading code proved toimpede using an efficient error correction system because theimplementation of an error correction system results in a decrease inprocessing capability as a result of the code period reduction andaccordingly of the number of available codes.

In addition, a study performed by J. L. Massey and T. Mittelholzer(Technical Assistance For The CDMA Communication System Analysis, FinalReport, Institute for Signal and Information Processing CH-8092 ETH,Zurich, September 1990) demonstrated that using preferentially phasedGold codes, i.e. Gold codes having minimum mutual cross-correlation,turns out to be an optimal choice for point-to-multipoint communicationnetwork. However, though preferentially phased Gold codes are a straightway to improve power with no reduction in the processing gain and withno bandwidth reduction in a usual asynchronous CDMA system (A-CDMA),this turned out to not be true for a synchronous quasi-orthogonalmultiple access system.

As a matter of fact, the coding scheme in such a system results in areduction in the number of available quasi-orthogonal spreading codesbecause the symbol rate is increasing for a given bandwidth, thusreducing the number of communication channels available.

These limitations make it difficult to use a code-synchronized CDMAsystem for commercial applications of a satellite communication system.For instance, for digital sound broadcasting or micro-terminal networks.

The object of the present invention is to overcome the limitationsimposed to the use of synchronized codes in a CDMA system and to thatpurpose there is provided a novel code division multiple accesstechnique.

This object is attained by virtue of the invention by a method and adevice for multiplexing signals for transmission at a given rate in aminimum frequency bandwidth through a Code Division Multiple Accesssatellite communication system, using compact low-power ground andon-board equipments.

In accordance with this invention, the incoming data signals are firstindividually encoded such that each of said signals is represented by anindividual symbol among an N-symbol constellation associated to Nseparate points, thus generating a bit stream in phase with the incomingsignal and a bit stream in quadrature with the incoming signal. Each ofsaid bit streams is assigned a signature in the form of an individualspreading code consisting in a binary sequence having a defined lengththat is preferentially derived from a preferentially phased Gold code.The individual spreading codes used for both bit streams arequasi-orthogonal codes. The bandwidth of the resulting signals is thuslimited by filtering and thereafter both baseband signals are applied toa conventional modulator in which they are used to modulate a carrier ina manner known per se. At the receive end of a link, the signal can beaccepted by a low-complexity digital receiver using a local replica ofthe afore-mentioned spreading codes to produce despread samples whichare suitable for being decoded. The carrier frequency and the code phaseare kept synchronous using any proper synchronization technique, exceptin the application of a point-to-multipoint communication as is the casein sound broadcasting because in such an application, thesynchronization is achieved automatically.

The system according to the invention provides the following advantages:

1. Highly efficient use of the available transponder power and bandwidthfor single and multiple beam satellites,

2. High transmission quality is achieved with mobile or portableterminals, whatever the propagation conditions may be, even in hostilepropagation conditions like multipath fading and shadowing,

3. Adaptability to highly variable traffic, i.e. equally efficient andlow traffic loads, and this practically without any performancedegradation in case of non-nominal conditions,

4. Flexibility to various applications: digital voice or soundbroadcasting, data transmission, mixed transmission,

5. Adaptability to various network configurations: star, multi-star,partially meshed networks.

6. Compliance with the power flux density (PFD) requirements for userterminals providing minimum RF interference with other services.

The multiple access system according to the invention is particularlyuseful in the field of digital broadcasting. Amongst other fields ofapplication it is worth mentioning micro-terminal satellitecommunication networks (Very Small Aperture Terminal VSAT networks) andcellular broadcasting systems.

The invention is described in more details in the following withreference to the accompanying drawings.

FIG. 1 is a block diagram of an exemplary multiplexing circuitry inaccordance with the invention.

FIG. 2 is a block diagram of an exemplary digital receiver.

FIG. 3 illustrates diagrammatically as an example a typical applicationof the multiple access system of the invention.

FIG. 4 is a diagram showing the performance of a system according to theinvention.

The basic concept behind the invention consists in associating eachinformation bit in the incoming data signal to an individual point in anN-point constellation. In practice, this concept is implemented in acircuitry essentially comprising a linear encoder followed an N-PSKmodulator. FIG. 1 shows a block diagram of an exemplary circuitry. Theencoder 11 receives the incoming bit stream DS and generates log₂ Nsymbols for each incoming bit. The output from the encoder is applied toan N-PSK modulator 12 according to a defined coding scheme with a symbolrate R_(s). The modulator output consists in two complex baseband bitstreams I and Q, having a symbol rate where R_(b) is the bit rate.

The way in which coded symbols are associated to the PSK constellationpoints is properly selected to optimize the system performance. Thetarget is to maximize the euclidean distance between the transmittedsignals. For this purpose a trellis-coding, for instance, is wellsuited. It is particularly worthy of note that simply modifying thecoding/modulation scheme as teached by this invention to performmultiple access, makes it possible to cope with a wide range ofparticular application requirements in order to increase the bandwidthand/or the spectral efficiency with minor or no modem hardwaremodifications.

For example, a 8 PSK constellation with a 8-states trellis code providesa 3 dB increase in efficiency with a binary error rate (BER) of 10⁻⁵,that is without decrease in capacity. Moreover, using such a codingscheme has the further advantage of improving insensitivity to nonlinear amplifier distortions and interference effects. Also, the errorsat the decoder output will occur in burst. This type of errors can becorrected easily in the decoder. More complex signal constellations,such as 16 PSK coded with r=2/3 for instance, will result in higherpower than when using an uncoded 8 PSK constellation with the samecapacity.

The signals at the output of modulator 12 are the in-phase I componentand quadrature Q component. These binary components are signedindividually in two multipliers 13 and 13' using two individualquasi-orthogonal spreading codes C_(I) and C_(Q) generated by a codegenerator 14 so as to produce two spread bit streams forming two blocks.These spreading codes are bit sequences having a length L. In order tominimize the mutual cross-correlation between the different users, thecode period L/Rc is preferably chosen equal to the symbol period 1/Rs.The modulator clock and frequency are controlled by a synchronizationsignal (known per se) of the communication system. This commonsynchronization signal for all the baseband signals is a precisereference which, if not modulated, is not affected by the datamodulation loss.

The length of the spreading codes C_(I) and C_(Q) is a major parameterwhich determines the system performance in terms of self-noise andspread chip timing jitter. In order to keep self-noise low, it isimportant to limit the timing jitter to within ±0.1 Tc (where Tc is thespread chip duration). A study made under contract for the Applicant hasdemonstrated that Gold code families having minimum mutualcross-correlation provide the optimal spreading code.

By using different generator polynomials for generating code families itis possible to generate a number of Gold code families showingquasi-orthogonal cross-correlation properties with codes inside the samefamily and pseudorandom correlation properties with codes of a differentfamily. This technique is of particular relevance in the case of amultibeam communication satellite for it is then possible to assign acode of different families to different beams thereby to reduce theamount of noise coming from adjacent beams.

Unfortunately, the number of families that can be generated is dependenton the code length. When using a short code, however, it is possible toincrease the number of codes by reusing a reverse version of eachsignature sequence. It can be shown that the new code exhibits the samecharacteristics as the Gold code and the same pseudorandomcross-correlation properties as the originating family.

The property of generating code sets is useful in a broadcast signal forinstance. As a matter of fact, it is possible to assign a givenquasi-orthogonal sub-set to the I arm and a different quasi-orthogonalsub-set to the Q arm. Isolation between the I and Q arms is guaranteedto be equal to the cross-correlation of random codes.

The spread bit streams I and Q at the output of the multipliers 13 and13' are multilevel signals. These signals are shaped in two shapingfilters 15, 15' and thereafter applied separately to modulators 16, 16'(known per se) in which they are used to modulate the in-phase andquadrature carrier components generated by a local generator 17. The twomodulated signals are then summed in a summing circuit 18 prior to beingconverted into the intermediate frequency bandwidth in a manner knownper se.

At the receiving end of a link, the signals are received in a lowcomplexity digital receiver. Various embodiments are possible. FIG. 2shows an exemplary block diagram. The receiver circuitry comprises asection operating at a chip rate Rc and a section operating at a symbolrate Rs.

Substantially the receiver comprises two distinct demodulation chainshaving a common input: one chain 301 for processing the master signaland the other chain 302 for processing the useful data signal. Aftercoherent baseband conversion in a demodulator 31, each signal isfiltered in a matched filter 32, and then digitized in a sampler 33 atthe rate of one sample per chip. Despreading the signal is thereafterperformed by multiplication of the samples with local replicas of thespreading codes C_(I) and C_(Q) in multipliers 34 which produce complexsample streams. The local replica code generator 38 is under phasecontrol of an acquisition and tracking logic unit 39 as outlined in"Chips Timing Synchronization in All-Digital Band-limited DS/SS Modem",R. De Gaudenzi, M. Luise, R. Viola, Proceedings of IEEE, InternationalComm. Conference ICC '91, Denver, Colo., U.S.A., Jun. 23-26, 1991.

After accumulation over one symbol period in accumulators 35, thedespread samples are sampled in a sampler 36 at the rate of one sampleper symbol. The resulting I and Q samples are then decoded in a basebandN-PSK Trellis demodulator 37 operating at symbol rate.

In order not to degrade the performance when extended Gold codes areused, the network terminals must be synchronized with an error of lessthan ±0.3 chip.

In case of a point-to-multipoint application (e.g. in soundbroadcasting), synchronization is performed by transmitting a referenceto all the terminals in the network. In the forward link, the userdemodulator 38 can make use of the central clock as a time reference forthe useful channel. In the return link, each spreading code is trackedindividually in the receiver. Frequency also has to be controlled quitetightly within a predetermined range (e.g. ±6.10⁻² Rb).

In a point-to-multipoint application, for instance in a microterminal(M-VSAT) network, a more sophisticated synchronization technique has tobe adopted. Generally, the synchronization technique proved not to becritical in practical applications. As a matter of fact, the chip rate(Rc) is normally kept low (<3 Mchip/s). Moreover, a few terminals beingout of synchronization does not provoke catastrophic consequences forthe network.

A variety of synchronization techniques may be used in aquasi-synchronous system. The presence of a reference available to allthe terminals in the network suggests the use of a master-slavesynchronization technique as described e.g. in "Telecommunication SystemEngineering", W. S. Lindsley and M. K. Simon, Prentice Hall EnglewoodCliffs N.J., 1973.

Master signal level monitoring at terminals provides a tool useful foropen-loop power control. Once the master code is correctly synchronizedat the user demodulator, the synchronization process continues with thetransmitter clock being corrected to compensate for propagation delayvariations.

One can distinguish between two types of closed loop synchronizationtechniques. In a centralized closed loop, synchronization at a remoteterminal is aided by a control station. In a local closed loop, theterminal autonomously provides alignment by receiving its own echolooped back by the satellite. The echo is compared with the localreplica and the error signal generated is proportional to the delayestimate. In order to reduce to a minimum the number of trackingcircuitries, delay tracking can be performed in closed loop mode. Thesynchronization error is determined at the hub station and transmittedto the mobile terminal using the communication channel. For fixed smallearth stations, synchronization is not critical and can be performed inlocal or centralized mode.

The multiplexing method according to the invention allows satellitecommunication systems to be realized, which are well suited for futurecommercial applications using low-complexity and low-cost modems to beimplemented at the terminal stations for performing the coherentdemodulation of the received signals with low loss (1-2 dB only).Moreover, thanks to a high spectral and energetic efficiency, the methodof the invention allows networks to be implemented which comprise smallterminal stations capable of handling data at a rate of some hundredsKb/s per channel, e.g. micro VSAT and mobile stations.

FIG. 3 diagrammatically depicts by way of example a typical network thatcan be implemented in carrying out the multiplexing method of theinvention. The network allows a great number of microterminals MT to beinterconnected with several hub stations VSAT by means of communicationchannels cooperating with a geostationary satellite GSAT. Control andcoordination of the network are achieved through a coordination hubstation NCS. Some microterminals MT may be mobile stations.Communications can be established, using the invention, with excellentperformances.

The performances that may be expected with the invention have beenanalysed by means of a simulation approach. FIG. 4 shows the variationof the probability of error as a function of the signal-to-noise ratiofor uncoded 8 PSK quasi-orthogonal CDM (8UQO-CDM), for uncoded 4 PSKquasi-orthogonal CDM (4UQO-CDM) and for 8-states trellis-codedquasi-orthogonal CDM (8 TCQO-CDM), for uncoded 4 PSK quasi-orthogonalCDM (4UQO-CDM) and for 8-states trellis-coded quasi-orthogonal CDM(8TCQO-CDM) for a processing gain of 15.5 with N=1, 7 and 14 (N is thenumber Of transmitters) as compared with the quasi-lower limits (QLB)calculated for N=1, 7 and 14. The results show that the performanceobtained is substantially constant whatever the spreading code lengthmay be.

It should also be emphasized that owing to the ease with whichsynchronization can be maintained between transmitter and receiverterminals, it is possible to implement low-complexity modems that areparticularly suitable to small terminals. Moreover, simple and compactterminal modems, that are well suited for a large scale integration, canbe used to advantage in implementing a digital sound broadcasting. Inthis typical application of a point-to-multipoint network, thesynchronization can be achieved by using a reference proper to thetransmission system itself, in particular the master code of the CDMAsystem as outlined in the foregoing.

It should be understood that the embodiments described in the foregoingare examples given by way of illustration and the invention is nowiselimited to these examples. Any modification, any variation and anyequivalent arrangement should be considered as being within the scope ofthe invention.

I claim:
 1. In a Code Division Multiple Access satellite communicationsystem, a method of multiplexing data signals to be transmitted on amodulated transmission carrier to a plurality of terminal stations byphase-shift keying carrier modulation, said method comprising the stepsof:prior to modulating the transmission carrier, individually encodingeach binary signal into an individual symbol pertaining to aconstellation of a finite number of symbols associated to the samenumber of distinct points so as to produce first and second bit streams,the first bit stream being in phase with an incoming signal and thesecond bit stream being in phase-quadrature with said incoming signal,and assigning to each of said two bit streams an individual signature byapplying an individual spreading code to such bit stream thereby toproduce two spread code to such bit stream thereby to produce two spreadcomposite bit streams intended each to being applied to a modulator, theindividual spreading codes being in quasi-orthogonal cross-correlationtherebetween.
 2. The method defined in claim 1, wherein the spreadingcodes are bit sequences derived from a preferentially-phased Gold code.3. The method defined in claim 2, wherein the spreading codes pertain toseveral code families, the codes in each family having quasi-orthogonalcross-correlation properties in relation to other codes of the samefamily and having pseudorandom correlation properties with codespertaining to a different family.
 4. A device for multiplexing datasignals comprising:a linear encoder arranged to accept data signals inan incoming bit stream from a data source and to generate coded bitstreams, a phase shift keying modulator having a finite number of statesand arranged to receive the coded bit streams from said linear encoderand to produce in response thereto, first and second distinct bitstreams, the first bit stream being in phase with the incoming bitstream and the second bit stream being in phase-quadrature with theincoming bit stream, a pair of combining devices connected,respectively, to an output of a separate phase shift keying modulatorfor combining each bit stream with an individual spreading code so as toproduce each a spread composite bit stream for modulating a transmissioncarrier, said two spreading codes being in quasi-orthogonalcross-correlation therebetween, and a spreading code generator arrangedto generate said two spreading codes.
 5. The device defined in claim 4,including a shaping filter connected to the output of each of saidcombining devices.